Behavior Ecology

Plenty of Fish in the OMZ

Logan, Ryan K., et al. “Patrolling the border: Billfish exploit the hypoxic boundary created by the world’s largest oxygen minimum zone.” Journal of Animal Ecology (2023).

Sailfish and blue marlins, also known as billfish for their long, sharp, sword-like bills, are some of the fastest fish in the ocean. These marine predators use unique foraging and diving techniques to prey on fish, squid and crustaceans in the enormous pelagic ocean.

The billfish’s prey are abundant, but because they are spread in a gigantic 3-D habitat they can be hard to find. So how do these bill fish manage to find the sparsely dispersed fish, squid and crustaceans in the ginormous ocean?

They use an invisible oceanic boundary created by a mass of water that contains almost zero oxygen!


The largest naturally occurring Oxygen Minimum Zone (OMZ) exists in the Eastern Tropical Pacific ocean (Figure 1). Here, the ocean surface (the first 25-50 meters) has uniform temperature and is saturated with oxygen from the atmosphere. But just below this lies a large layer of water containing zero dissolved oxygen spanning 200-1500 meters below the ocean surface!

Figure 1. Global Oxygen Minimum Zones (from Moffitt, S. E. et al., PloS one, 2015).

Where are the fish?

The transition to the OMZ creates an oceanic ‘front’ where the oxygenated surface waters meet the deoxygenated waters below (also known as the oxycline). This ‘front’ is also defined by the thermocline, which is where the water temperature drastically changes with depth. The lack of oxygen (hypoxia) and low temperatures in the OMZ limit the distribution of billfish prey to the oxygenated surface waters – this phenomenon is known as hypoxia-based habitat compression.

Figure 2. A depiction of the oxycline and thermocline in the Eastern Tropical Pacific OMZ.

During the day, when it is easy to see in surface waters and when the predatory billfish are active, their prey try to escape predation by swimming to the darker waters at the boundary of the OMZ. This increases prey density at the oxycline, making them easier targets for the billfish to capture and feed on!

In this study, Logan et al. tagged and released billfish to monitor their predatory behaviors at the boundary of the OMZ. They observed a behavior that has never been seen in such predators where the billfish dive multiple times below the OMZ boundary/thermocline and subsequently attack the prey concentrated at the boundary from below (Figure 3)! This behavior allows billfish to increase the number of prey they encounter while conserving energy.

Figure 3. Excerpt of sailfish diving behavior from video footage. Bottom graph shows the dive path over time on the x-axis and depth on the y-axis; the dive path is color-coded to show oxygen saturation in the water. The horizontal gray line indicates the hypoxic boundary.

Ocean Deoxygenation

 Although OMZs are naturally occurring regions in the ocean, they are becoming larger both in the vertical and horizontal directions due to anthropogenic climate change. Hypoxia-based habitat compression is now occurring at abnormal rates and is leading to habitat loss.

Figure 4. Illustration of the expanding OMZ and associated habitat compression.

Not only will this rapid change affect biodiversity, it will have a large impact on the fishing and aquaculture industry – which is why it is imperative to conduct studies such as this on to understand how marine predators and prey alike will largely be affected by deoxygenation and the expansion of OMZs.

For more information about ocean deoxygenation:

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